Add vnode_management_timer to configuration

From CSE kanban: The [app.config](http://docs.basho.com/riak/latest/references/Configuration-Files/#app-config) section of our [Configuration Files](http://docs.basho.com/riak/latest/references/Configuration-Files/) documentation needs a mention of the `vnode_management_timer` setting which lives in the **riak_core** section of `app.config`.

See the [1.1.2 Release Notes](https://github.com/basho/riak/blob/master/releasenotes/riak-1.1.org) for more information on `vnode_management_timer`.